Rechargeable lead-acid batteries, nickel cadmium (NiCd) batteries and other similar batteries are widely used in a variety of applications, from children's toys to video cameras. Rechargeable batteries are also necessary for powering electric vehicles. Because of clean air concerns, electric vehicles may be the most important future application for rechargeable batteries. Unfortunately, present day batteries used in prototype electric vehicles are limited in their ability to maintain an adequate charge over a period of extended use and subsequently require long charging times. Thus, there is a clear need to develop new and more efficient ways to recharge batteries and, more particularly, to reduce charging times.
Two basic processes are now used to charge and recharge batteries. The first process involves supplying a constant voltage to the chargeable battery. The second process involves supplying a constant current to the chargeable battery. When lead-acid batteries used in electric vehicles are recharged using these conventional processes, the recharging times are of the order of ten hours.
Considerable research effort is being directed to the development of procedures and equipment to reduce battery recharging times. Various rapid charging processes have been developed. One process is based on the concept of internal resistance-free voltage and use of interrupting circuitry for programmed interruption of electrical charging power to batteries undergoing charge. Preselected time intervals are employed to permit the detection of the internal resistance-free voltage. The power delivered to the battery is reduced when the resistance-free voltage exceeds the preselected reference voltage. The rate of charge is reduced gradually to maintain internal resistance-free voltage in the battery at preselected reference voltages throughout the charging process.
Another process involves a pulse charging methodology for rapid charging of batteries used in electric vehicles. This process involves one or more depolarization pulses that are believed to alter ion migration between electrodes in ways that allow charged ions to more efficiently interact with the electrode plate. As a result, heat production during charging is minimized. This technique stirs the ions in the battery's electrolyte by applying alternating positive and negative pulses which helps to reduce the electrode polarization and increase charging efficiency.
Present rapid charging techniques suffer from a number of significant problems. While known rapid-charge algorithms take advantage of the high-charge acceptance during initial charging, recharging a battery that has been almost fully discharged requires as much time to charge the battery from 85% to 100% state of charge (SOC) as was required using conventional (i.e., non-rapid charging) techniques. This is because rapid charging produces an undesirable amount of heat and gas, which can result in battery failure, unless the "rapid charge rate" is reduced during the last 15% of the charging process. The net result is that the last 15% of battery capacity can not be made available with rapid charging techniques due to the reduction in the charge rate necessary to avoid overheating and gas production. This fails to meet the goal of full capacity recharging, and prolonged undercharging also results in a sharp decrease in battery life and capacity. In addition, currently available rapid charging techniques involve complex and expensive recharger infrastructure.
Thus, there is a clear need in the industry for an alternative battery charging system and method to enhance battery charging and reduce the time necessary for recharging, while at the same time reducing the costs and complexities associated with current charging techniques.